With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
A typical gas turbine engine has a substantial number of electrical components which serve, for example, to sense operating parameters of the engine and/or to control actuators which operate devices in the engine. Such devices may, for example, control fuel flow, variable vanes and air bleed valves. The actuators may themselves be electrically powered, although some may be pneumatically or hydraulically powered, but controlled by electrical signals.
Electrical power and signals to and from the individual electrical components are commonly transmitted along conductors. Conventionally, such conductors may be in the form of wires and cables which are assembled together in a harness. In such a conventional harness, each wire may be surrounded by an insulating sleeve. The connections between the individual components and the conventional harness are made, for example, by multi-pin plug and socket connectors. Similarly, communication between the harness and power, control and signalling circuitry is achieved through a multi-pin connector.
By way of example, FIG. 2 of the accompanying drawings shows a typical gas turbine engine including two conventional wiring harnesses 102, 104, each provided with a respective connector component 106, 108 for connection to circuitry accommodated within the airframe of an aircraft in which the engine is installed.
The harnesses 102, 104 are assembled from individual wires and cables which are held together over at least part of their lengths by suitable sleeving or braiding. Individual wires and cables, for example those indicated at 110, emerge from the sleeving or braiding to terminate at plug or socket connector components 112 for cooperation with complementary socket or plug connector components 114 on, or connected to, the respective electrical components.
Each conventional harness 102, 104 therefore comprises a multitude of insulated wires and cables. This makes the conventional harness bulky, heavy and difficult to manipulate. It is desirable to reduce the size and weight of components on gas turbine engines, particularly, for example, gas turbine engines for use on vehicles, such as aircraft.
It is proposed to replace conventional harnesses with flexible printed circuit board harnesses (FPCB harnesses). An example of a portion of a flexible printed circuit board harness 120 is shown in FIGS. 3 to 6. FIG. 3 shows a perspective view of an FPCB harness portion, and FIGS. 4, 5, and 6 show side, top, and cross-sectional views respectively.
The FPCB harness 120 typically comprises a flexible (for example elastically deformable) substrate 140 with conductive tracks 130 laid/formed therein. The FPCB harness 120 may thus be deformable. In the example shown in FIGS. 3 to 6, the FPCB harness 120 extends along a length in the x-direction, a width in the y-direction, and a thickness (or height or depth) in the z-direction. The x direction may be defined as the axial direction of the FPCB harness. Thus, the x-direction (and thus the z-direction) may change along the length of the FPCB harness 20 as the FPCB harness is deformed. The x-y surface(s) may be said to be the major surface(s) of the FPCB harness. In the example shown in FIGS. 3 to 6, the FPCB harness is deformable in the z direction, i.e. in a direction perpendicular to the major surface. FPCB harnesses may be additionally of alternatively deformable about any other direction, and/or may be twisted about any one or more of the x, y, or z directions.
The flexible substrate 140 is typically a dielectric, and the conductive tracks 130, which may be surrounded by the substrate, are typically formed using a suitable conductive material. The conductive tracks 130 may be used to conduct/transfer electrical signals and/or electrical power, for example around a gas turbine engine and/or to/from components of a gas turbine engine. The size (for example the cross-sectional area) and/or the shape of the conductive tracks 130 can depend on the signal to be transmitted through the particular conductive track 130. Thus, the shape and/or size of the individual conductive tracks 130 may or may not be uniform in a FPCB harness 120.
The example shown in FIGS. 3 to 6 has two layers of conductive tracks 130 running through the substrate 140, with three tracks in each layer. However, the number of layers and the number of tracks in the FPCB harness 120 can vary.
Using an FPCB harness to transmit electrical signals and/or power can be advantageous over a conventional harness, for example because of its reduced size, weight and/or complexity.
A lightning strike on an aircraft causes current to pass through the exterior skin and structures of the aircraft. This current can induce transients, known as indirect lightning, in components such as FPCB harnesses. For example, a 200 kA direct lightning strike on an engine can induce up to about 1500 V (overvoltage) to a harness. This can result in high voltage differentials between tracks which may produce damage in the harness.
IPC gives standard track spacing rules (see IPC 2223B, May 2008, Sectional Design Standards for Flexible Printed Circuit Boards, Section 6.3 (electrical clearances)) between tracks of FPCBs to account for normal conditions, but excludes indirect lightning strikes. In particular, IPC 2223B gives a table of electrical voltages against spacing distances. According to this table, above a 500 V differential the distance between tracks increases significantly. Thus, following IPC standard spacing rules in an FPCB harness which may have to accommodate a potential voltage differential of 1500 V during indirect lightning strikes can lead to the spacing between conductors becoming large, with the result that the harness may become unfeasibly large, heavy and/or expensive.